Temperature and Urea Concentration as Drivers of Gaseous Ammonia Production from Manure: Mechanisms, Interactions, and Modeling Implications

Abstract

Ammonia (NH₃) emissions from livestock manure pose significant environmental and public health risks, yet predictive models often oversimplify the complex interactions between environmental and substrate drivers. This review synthesizes current evidence on the roles of temperature and manure urea concentration as drivers of NH₃ production, focusing on a foundational in vitro microchamber study that utilized a 4 × 3 factorial designs (urea at 0, 2.4, 6.0, and 9.6 g/kg; temperatures of 25, 30, and 35 °C). The experimental data demonstrate that temperature exerts a dominant influence on production rates, with increases of 3.9–5.2 mg NH₃/m²/min at 30 °C and 7.5–10.0 mg NH₃/m²/min at 35 °C relative to a 25 °C baseline. While higher urea loads initially elevate emission rates, production plateaus at 9.6 g/kg, reflecting urease saturation or substrate inhibition. Crucially, a significant temperature–urea interaction (P < 0.001) reveals that temperature sensitivity is highly dependent on substrate availability. Furthermore, urea hydrolysis immediately elevates manure pH by approximately 1.2 units, creating a positive feedback loop that shifts the NH₃/NH₄⁺ equilibrium toward the volatile form. Integrating these mechanistic insights with independent evidence across dairy excreta and soil systems confirms that the rate-limiting step shifts dynamically across the temperature–urea domain. Consequently, current process-based emission models, which frequently apply uniform temperature correction factors and rely on static pH inputs, risk systematic miscalculations. To accurately predict emission hotspots, models must adopt conditional temperature coefficients and explicitly represent urease capacity limitations, dynamic pH coupling, and patch-scale spatial heterogeneity. These structural refinements are essential for translating controlled laboratory findings into accurate, policy-relevant national emission inventories.